Transmission loss of an optical fiber has a significant impact on the performance of the optical communication network. As a material of an optical fiber, silica glass has been widely used. The transmission loss of an optical fiber in a wavelength region around 1,550 nm employed for usual optical communication network, is considered to be mainly due to Rayleigh scattering of silica glass.
It is known that the higher the fictive temperature of silica glass, the higher the Rayleigh scattering coefficient of the silica glass. Thus, as a method for producing an optical fiber, a method of lowering the fictive temperature of silica glass to lower the Rayleigh scattering coefficient thereby to reduce the optical transmission loss has been proposed (Patent Document 1).
Further, a method of adding a very small amount of e.g. an alkali metal to silica glass to shorten the structure relaxation time of the glass thereby to reduce the optical transmission loss has been proposed (Patent Document 2).
Silica glass is used for an optical member such as a glass substrate or a lens for lithography employing ultraviolet rays.
It is known that the transmittance of silica glass is reduced by irradiation with high energy ultraviolet rays. As a method for preventing a decrease in the transmittance of silica glass by ultraviolet rays, a method of lowering the fictive temperature of the silica glass has been known (Patent Document 3).
Patent Document 4 discloses that the hardness of silica glass can be increased by heating and pressurizing it in a specific atmosphere.